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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

310
ALASKAN GLACIER STUDIES
"My attention has been directed by Gannett to the fact that several of the cascading glaciers make two leaps, and that there is a certain amount of harmony in the spacing of the falls. When the region shall have been thoroughly studied it is possible that the interpretation of these correspondences may develop a special chapter in the history of the ice retreat.
"With the aid of a series of photographs made by Merriam, I have computed the approximate heights of the more important cascades, as follows: Wellesley, 1700 feet; Vassar, 2200; Bryn Mawr (trunk), 1300, (left branch) 2700, (right branch) 2500; Smith, 1250, 1700 and 2600; Radcliffe, 1800 and 3500. When these are platted to scale in then* proper vertical and horizontal relations they fall into two series, descending
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FIG. 41.   HANGING VALLEYS ASCENDING NOBTHWAHD IN COLLEGE FIOBO APTBR G. K. GILBEHT.
southward from the head of the fiord. Making some allowance for the greater volume of the side glaciers when the trunk glacier filled the fiord, I have indicated the profile of the trunk glacier by a dotted line (AB) . The inclination of this line from the horizontal is about 2, or one in twenty-five. Its height above tide ranges from 2800 to 4800 feet, and it indicates a thickness of ice exceeding these figures by the depth of the fiord, whatever that may be. In the line of Gannett's suggestion, a second tentative profile (CD) is drawn in similar relation to the crests of the lower series of cascades.
"The depth of ice indicated by the hanging valleys is somewhat less than that which would be inferred from the rounding of projections, and it seems probable that the epoch during which the hanging valleys received their principal sculpture was not the epoch of mp^TniiTn glaciation."
Submarine Topography.   The soundings made in College Fiord in the summer of
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FIG. 42.   NATURAL SCALE CROSS-SECTION or COLLEGE FIOHD.
1910 enable us to carry the discussion of glacial erosion farther than was previously possible (PI. CXXVI). The depth of water near the fronts of Harvard and Yale Glaciers, 636 and 282 feet respectively, proves that neither of those glaciers is afloat, and that they must now be eroding on the fiord bottom. The line of rock ledges visible under the edge of Yale Glacier shows that in the past erosion has been less active in mid-fiord than on either side.
The water along the axis of College Fiord varies in depth from 174 to 804 feet, and the cross-sections of the fiord show that it has the typical U-shape (Fig. 42) below sea level. The longitudinal slope away from Harvard Glacier is interrupted near the terminus of the Smith Glacier where the depth decreases to 306 feet, by another swell